PURPOSE: To impartially give transmitting right to both upstream and downstream sides with a simple hardware constitution by dividing cells which make transmission to an incoming bus from a network terminating device into two kinds of upstream- and downstream-side preferential cells and alternately allowing both sides cells to make transmission.

CONSTITUTION: Plural terminal adapters 14 are connected in parallel with each other between an incoming and outgoing buses from a network terminating device 11 of a terminal system of a wide-band ISDN and continuous cells generated by the frame generating device 12 of the device 11 are alternately sent to the incoming bus from a priority assignment circuit section 13 by dividing the cells into upstream- and downstream-side preferential cells. The adapters 14 receive the alternately sent preferential cells by means of their reception I/Fs and a priority discriminating section 15 discriminates the upstream-side preferential cells from the downstream-side preferential cells and immediately sends the upstream-side preferential cells to a data transmitting section 17. Regarding the downstream-side cells, a transmittable state discriminating section 16 checks the cells whether or not they have request passes from the outgoing bus and, only when the cells have no transmission request, the section 16 can send a data transmission request to the section 17.